Study design and restrictions due to the fact that the study participants are suffering from dementia
The study had several limitations mostly because of the inherent difficulties in investigating people with dementia.
The simple, time-efficient and often used Mini Mental State Examination (MMSE)  is considered as a suitable instrument to assess cognitive impairment in medical practice. It can be used as a basic diagnostic tool for the preliminary quantification of cognitive deficits and as assessment of their severity  but the sensitivity is questionable, especially in mild dementia. The use of the MMSE, instead of more comprehensive testing procedures, may have led to inaccuracies in the grouping of some subjects.
The degree of dementia and the setting during the evaluation (e.g. clinic vs. long-term care facility) may influence the compliance and cooperation of the subjects during the oral health assessment.
It can be assumed that some measurements (e.g. detection of caries and periodontitis) are more prone to misinterpretation than others (e.g. Oral hygiene index (OHI)). Experience shows that measurement procedures that are associated with patience and the acceptance of inconvenience (e.g. probing of pocket depth) are not well tolerated with the increase in cognitive impairment. To the authors best knowledge there are no studies investigating the reliability of the used measurement instruments in people with dementia or different evaluation settings (dental chair vs. examination in a long-term care facility without a dental chair and its light source, most of the subjects sitting upright or lying in bed etc.).
Although the investigator was a dentist experienced in patients with dementia, not all measurements could be completed during the evaluation due to incompliance or other unfavorable circumstances.
As a result, some parameters in the analyzed data set showed expectation values that were too low, which allowed a statistical evaluation only on a purely descriptive basis (e.g. Periodontal Screening Index (PSI), Bleeding on Probing (BOP)).
Most subjects with mild, moderate or severe dementia are long-term care residents, required comprehensive care and had been examined in the long-term care facility. For the recruitment of subjects with advanced dementia, we have had a limited number of long-term care facilities that cooperate with both the inpatient and mobile dental clinic (mobiDent™). Subjects without dementia or with mild cognitive impairment, living at home, were recruited within the clientel of the dental clinic and examined there.
Utilization of dental services
The utilization of dental services was found to be dependent on the degree of dementia. With the increase in dementia, there was a significant reduction in the demand on the services of the dentist and dental hygienist.
Despite the possibility for the uptake of dental services with the mobile dental clinic for subjects of this study living in long-term care facilities, the utilization of dental services by a dentist and especially by a dental hygienist decreased significantly with increasing dementia. It can therefore be assumed that the utilization of dental services by people with dementia depends more on the cognitive abilities, the need for care, the living situation and the oral functional capacity than on the possible access to dental services. The influence of these and other factors on the use of dental services by people living in long-term care facilities should be observed in further studies to adapt the concepts of mobile dental services offered.
Chen et al. (2013) showed that oral health measurements are poor in people with dementia independent of their residential status. They assumed that oral health had declined before the change in the living situation . Therefore our recruiting process should not have any influence on the results. Warren et al. (1997) conclude that it is unclear when the effects of a cognitive decline, the loss of social support and the functional decline cumulate and combine as risk factors for a deteriorated oral health. Furthermore, this effect may arise only in later stages of dementia and shows an individual variety .
In our study there was a significant difference in the number of teeth that are decayed, but not in the number of filled or missing teeth or the DMF/T index itself with increasing cognitive impairment or dementia. A dependency was found between the degree of restoration and dementia, which decreases with increasing dementia. Oral and denture hygiene deteriorated significantly with the increase in dementia. Nevertheless, periodontal therapy was required for all subjects independent of their degree of dementia while bleeding on probing was increasing with increasing dementia.
There was no difference in the DMFT index and the number of missing or filled teeth while a significant difference in the number of decayed teeth was observed with regard to the different dementia groups in this study. In the literature no differences in the DMFT mean values are described also [36, 37]. An explanation for this finding might be that people with dementia received the same dental treatment as people without dementia before the onset and progression of the disease and the accompanied increase in need of care. Nevertheless, Ribeiro et al. report a higher DMFT in subjects with Alzheimer`s disease . Furthermore, caries is more common in people with dementia [13,14,15] which confirms our findings. A cause for this finding might be the increasing difficulties or inability to treat people with progressed dementia chairside. Often only treatments under general anaesthesia (GA) are possible . If relatives/legal representatives, medical or financial circumstances do not allow a treatment under GA this might result in a higher number of decayed teeth in people with dementia.
The proportion of subjects with moderate and severe periodontitis was high in this study and did not differ with regard to the degree of dementia. Several studies in the literature confirm our results by showing no significant differences in gingival or periodontal diseases between people with and without dementia [36, 40,41,42,43,44]. An explanation might be that the prevalence of progressed periodontal diseases is high in older patients. Periodontal diseases might therefore have been established in people with dementia before the onset of the disease which complicates the evaluation of the influence of dementia on periodontal diseases. It is conceivable that oral health, in general, might have already deteriorated in the frailty phase of life, regardless of cognitive impairment. On contrary, De Souza et al. (2014) point out that people with dementia suffered more from periodontal disease than people without dementia .
With increasing dementia, Bleeding in Probing (BOP) increased significantly in this study as described before by Maldonado et al. (2017) .
The Oral Hygiene Index (OHI) and its components (Debris Index (DI), Calculus Index (CI)) by Greene and Vermillion increased with the increase in dementia and differed highly significantly between the dementia groups in this study. In the literature there are studies using the OHI index by Greene and Vermillion which report a Debris Index (DI) of 2.1, a Calculus Index (CI) of 2.0 and an OHI of 4.5. in subjects with dementia [38, 47, 48]. which results in a better oral hygiene of the subjects than in our study (mDem: DI Median 3.2 (Range 0.5–5.5), CI Median 3 (Range 0.25–4.5), OHI Median 6.2 (Range 0.75–9.5); modem: DI Median 4 (2–6), CI Median 3 (1.3–6), OHI Median 7.2 (Range 3.3–12); sDem: DI Median 4 (Range 2.3–6), CI Median 3.3 (Range 2–6), OHI Median 7 (Range 4.8–12)). Furthermore, Warren et al. (1997) observed no significant differences between dementia subtypes and healthy controls for the DI but investigated significant differences in the DI between people with moderate to severe dementia compared to people without dementia .
The denture hygiene, which was assessed visually without staining by the dentist, deteriorated significantly with the increase in dementia in this study. Zenthöfer et al. (2014) observed no differences in the denture hygiene between people with and without dementia measured with the Denture Hygiene Index (DHI) . Due to the nature of the measurements in the study of Zenthöfer et al. (2014) and ours no comparison of the results are possible. Since in our study, the utilization of dental services was lower for people with dementia, this might explain the worse denture hygiene observed. Also, a bias cannot completely be ruled out since the investigator in this study was not blinded. Furthermore, the visual judgement has limitations in differentiating between different hygiene status`.
The degree of restoration decreased significantly with the increase in dementia which might be a consequence of the reduced utilization of dental services. Also, one must consider that dental treatments in patients with dementia and an associated reduced Oral functional capacity (OFC) might have be limited due to reduced compliance or the avoidance of treatments under general anaesthesia, for example. In the literature no findings about the degree of restoration in people with dementia can be found to the authors best knowledge.
Since periodontal disease were common in all subjects independently of their cognitive status, but BOP, as an acute sign of inflammation, and the number of decayed teeth was higher in people with dementia, a higher impact of the daily oral hygiene might be considered as an explanation. Therefore, the authors suggest to substantially improve daily oral hygiene of people with dementia to reduce plaque and the resulting oral diseases (periodontitis, caries). A prerequisite for this is a structured training of nursing staff and relatives who are involved with daily oral and denture hygiene. This also includes intensive practical exercises which should be carried out by dental professionals. This requires structured concepts, which might also need to be co-financed by the health insurance companies as part of a prevention promotion program. In addition to improving the training and further education of nursing staff, individual hygiene concepts as well as education in nutritional aspects (less cariogenic food, reduced frequency of (finger)food etc.) that a dentist or dental hygienist could develop and implement with nursing staff, carers and relatives should also be financially supported by third parties (e.g. health insurance companies) since the deterioration in oral health seems to be mainly a consequence of the cognitive decline.
Interdisciplinary research cooperation’s should be strengthened in order to facilitate recruitment processes and improve research in people with dementia.